Fermi-LAT detection of a historic gamma-ray flare from OQ 334 with ASAS-SN optical brightening

ATel #17821; Sandeep Kumar Mondal (Tsung-Dao Lee Institute, Shanghai Jiao Tong University), Avik Kumar Das (Instituto de Astronomia - UNAM (IA-UNAM)), Amar Deo Chandra (Aryabhatta Research Institute of Observational Sciences)
on 29 May 2026; 12:51 UT
Credential Certification: Amar Deo Chandra (amar.deo.chandra@gmail.com)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray flaring activity from a source positionally consistent with the flat-spectrum radio quasar (FSRQ) OQ 334 (B2 1420+326), also known as 4FGL J1422.5+3223 (Abdollahi et al., Fermi Large Area Telescope Fourth Source Catalog, ApJS, 247(1):33, 2020). The source coordinates are R.A. = 215.626579 deg and Dec. = 32.386233 deg (J2000; Xu et al. 2019, ApJS, 242, 5), and redshift, z = 0.6819 (Hewett & Wild, MNRAS, 405(4):2302-2316, 2010). This FSRQ was recently found to be flaring brightly in the radio, NIR, optical and MeV-GeV gamma-ray bands (see, e.g., ATels #17802, #17795, #17792, #17790, #17782, #17745, #17654).

Preliminary analysis indicates that the source underwent its highest gamma-ray state (till date) on May 15, 2026 (MJD 61175.16), with daily averaged gamma-ray fluxes (E>100 MeV) of (4.69 ± 0.26) × 10^-6 ph/cm²/s (statistical uncertainty only). This corresponds to a flux enhancement by a factor of more than 57 relative to the average flux reported in the data release of the fourth Fermi-LAT source catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.11±0.05, indicating a harder spectrum than the 4FGL-DR4 value of 2.32±0.01. The source also underwent another flaring event on May 25, 2026 (MJD 61185.16), with daily averaged gamma-ray flux (E>100 MeV) of (3.30 ± 0.27) × 10^-6 ph/cm²/s (statistical uncertainty only). This corresponds to a flux enhancement by a factor of more than 40 relative to the average flux reported in 4FGL-DR4 with a spectral index of 2.00±0.06.

We also noticed an optical flux enhancement around May 13, 2026 (MJD 61173.37), in the ASAS-SN survey (g-band magnitude of 13.704±0.012), followed by another flux-enhancement state, reported on May 27, 2026 (MJD 61187.30), with a magnitude of 13.993±0.024.

Fermi normally operates in an all-sky scanning mode, hence regular gamma-ray monitoring of this source will continue. OQ 334 is listed in Fermi-LAT daily monitored source list, a preliminary light curve is available at OQ 334 . The light curve of this source can also be accessed through the Fermi Light Curve Repository at 4FGL J1422.3+3223 . We encourage multifrequency observations of this source.

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.